1. Field of the Invention
The present invention relates generally to the field of compressed fluids and pressure vessels for compressed fluids, and more particularly, to a pressure vessel, and method of using same, with a flexible or inflatable liner that can be inserted within the vessel body for providing a hermetic barrier between inner surfaces of the vessel body and stored gases and that can later be removed to facilitate reuse of the pressure vessel.
2. Relevant Background
For many years, pressure vessels have been used for storing gases for scientific and industrial uses. A well-known example of such a pressure vessel is the cylinders used for storing compressed gases, such as helium, oxygen, nitrogen, and other gases. These compressed gas cylinders or pressure vessels typically have a body that is about a foot in diameter and four to five feet long with a flattened bottom and a reduced top end or neck that has a valve screwed into inner threads to provide a hermetic seal. The pressure vessel bodies are ordinarily constructed of metal, such as carbon steel, to economically achieve the high strengths needed to contain gases at higher pressures, e.g., in the range of 2000 to 5000 pounds per square inch.
While the use of pressure vessels for storing gas is well known, there is a growing and continuing need for improvements in safely, inexpensively, and effectively storing gases while retaining gas purities. Scientific, medical, and manufacturing processes increasingly require very pure gases to be stored and later delivered according to stringent specifications. For example, in the ultra-high technology production processes for making computer chips where the transistor size is on the order of microns, the specifications on gas purity are extremely demanding.
Gas purity is often degraded when the stored gas directly contacts the interior surfaces of the vessel or container body as contaminants on interior surfaces mix with the gas. This contamination problem can be exasperated by surface imperfections on the inner surfaces of the vessel body. The gas purity may also be degraded if the stored gas is not compatible with the material or metal used for fabricating the vessel body. In these situations, purity is degraded by the release of contaminants produced in chemical reactions occurring between the body of the pressure vessel and the stored gas. In an attempt to control contamination, a pressure vessel may be internally polished at original manufacture and during periodic maintenance and is typically cleaned, e.g., steam cleaning, pressure cleaning, chemical cleaning, and the like, according to exacting standards. While these techniques may provide useful control over contaminants these processes are labor and equipment intensive and are often costly. Further, the pressure vessel may need to be cleaned again prior to reuse or even polished based on the age of the vessel, and these techniques do not address the issue of material incompatibility between stored gases and vessel body materials. In many cases, a pressure vessel simply cannot be used for other gases after it has been used for a gas that is incompatible with the proposed new gas, which severely limits the recycling or continued use of existing pressure vessels or compressed gas cylinders.
Pressure vessels, such as compressed gas cylinders, with rigid liners made of corrosion-resistant metal plating (such as nickel plating), glass, ceramic, plastic, or other material have been developed in an attempt to reduce control gas purity degradation and to minimize material compatibility problems. While such rigidly lined vessels are much more effective at providing consistently pure gas, there are a number of issues that arise from using lined pressure vessels. Lined pressure vessels are often very expensive to fabricate. For example, metal plated or lined vessels are typically produced using an electroplating process with a relatively expensive corrosion-resistant metal, and the use of ceramic or other material liners typically requires baking, e.g., vacuum baking, the entire vessel. Post-production steps generally include polishing and/or extensive cleaning of all interior surfaces. Alternatively, the manufacturing may be performed entirely or partially in a clean room or clean environment. Clearly, the manufacture of lined pressure vessels is more expensive in labor, material, and equipment costs than standard unlined pressure vessels.
There are also service problems with using rigidly lined pressure vessels for storing compressed gases. For example, there are often problems obtaining an adequate seal between the neck and the valve due to the difficulty in cutting threads in liners and efforts continue to provide an effective solution to this sealing problem. Another problem is that the surface of the liner itself can have irregularities and high surface roughness, such as due to electroplating processes, that can trap contaminants that could later be released degrading the purity of stored gas. Pressure vessels with rigid liners, such as metal-plated cylinders, have also proven to have limited durability, especially during rigorous retesting procedures and during service conditions that may create differing expansions in the liner and the adjacent vessel body or that applies large internal pressures mainly on the liner. This has led to a shortened service life as once flaws in the liner develop the pressure vessel is typically not repaired but is instead taken out of service.
Hence, there remains a need for an improved pressure vessel that maintains the purity of stored gases and addresses the problem of material incompatibility with stored gases and inners surfaces of vessel bodies and with stored gases and previously stored gases. Preferably, such an improved pressure vessel would be cost effective to manufacture, to use, and to maintain, and would be compatible with existing systems that utilize pressure vessels, such as compressed gas cylinders, and with the existing pressure vessel fleet, e.g., provide a technique for continued use of previously manufactured and in-use vessels. Further, it is desirable that such pressure vessels comply with existing and future regulations covering storage of compressed gases, such as those issued in the United States by the Department of Transportation (DOT).